Wireless communication system, base station, mobile station, and wireless communication method

ABSTRACT

A wireless communication system controls a transmission timing in each cell so that a control channel of a first cell and a data channel of a second cell temporally overlap each other. The wireless communication system includes a base station of the first cell. The base station of the first cell includes a first control unit and a first communicating unit. The first control unit notifies a base station of the second cell of information used to specify a resource of the control channel of the first cell. The first communicating unit transmits a control signal to a mobile station of the first cell by using a first resource of the control channel of the first cell.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of InternationalApplication PCT/JP2011/051948, filed on Jan. 31, 2011, and designatingthe U.S., the entire contents of which are incorporated herein byreference.

FIELD

The embodiments discussed herein are directed to a wirelesscommunication system, a base station, a mobile station, and a wirelesscommunication method.

BACKGROUND

Conventionally, for LTE (Long Term Evolution) or LTE-Advanced as a nextgeneration mobile communication system, a heterogeneous network has beenstudied in order to improve system capacity and coverage. Theheterogeneous network is a network in which a macrocell and a cell of abase station with low transmission power (hereinafter, described as a“picocell”) are arranged so as to coexist with each other. In such anetwork, when the macrocell and the picocell are operated at the samefrequency, interference from the macrocell to the picocell becomes aproblem. Namely, in a mobile station connected to a base station of thepicocell (hereinafter, described as a “pico base station”), a signalfrom the pico base station is interfered with by a signal from a basestation of the macrocell (hereinafter, described as a “macro basestation”).

The inter-cell interference as described above influences thecommunication quality in each of physical channels (a control channeland a data channel). In particular, in a system in which a transmissiontiming of a subframe is synchronized between cells, the inter-cellinterference may occur between the control channels and between the datachannels. As a technology for reducing the inter-cell interference asdescribed above, there is a technology for shifting a transmissiontiming of a subframe in the pico base station with respect to atransmission timing of a subframe in the macro base station, in an OFDM(Orthogonal Frequency Division Multiplexing) symbol unit. In such atechnology, a data channel of the macro base station that overlaps acontrol channel of the pico base station in the time domain as a resultof the shift is overwritten (muting) with a null symbol withtransmission power of zero.

-   Non Patent Literature 1: 3GPP TR 36.814 V9.0.0 (2010-03)-   Non Patent Literature 2: 3GPP TS 36.211 V8.9.0 (2009-12)-   Non Patent Literature 3: 3GPP TS 36.213 V8.8.0 (2009-09)-   Non Patent Literature 4: 3GPP R1-103227

However, in the technology described above, while interference from thedata channel of the macro base station to the control channel of thepico base station is reduced, there is a problem in that the receptioncharacteristics of the data channel of the macro base station isdegraded. Namely, because a null symbol is set in a resource that ismuted to reduce the interference among resources of the data channel ofthe macro base station, the amount of data that the macro base stationcan transmit per unit time is reduced accordingly. The reduction in theamount of transmission data is a cause of degradation of the receptioncharacteristics of the data channel.

SUMMARY

According to an aspect of the embodiments, a wireless communicationsystem controls a transmission timing in each of cells so that a controlchannel of a first cell and a data channel of a second cell temporallyoverlap each other. A base station of the first cell includes a firstcontrol unit that notifies a base station of the second cell ofinformation used to specify a resource of the control channel of thefirst cell, the resource corresponding to a predetermined resource unit;and a first communicating unit that transmits a control signal to amobile station of the first cell by using a first resource of thecontrol channel of the first cell, the first resource corresponding toat least a part of the predetermined resource unit and serving as adecoding object of the mobile station of the first cell. The basestation of the second cell includes a second communicating unit thattransmits a null symbol by using a second resource of the data channelof the second cell, the second resource corresponding to thepredetermined resource unit. The mobile station of the first cellincludes a third communicating unit that receives the control signaltransmitted by the base station of the first cell via the first resourceand receives the null symbol transmitted by the base station of thesecond cell via the second resource.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating an example of a heterogeneous network;

FIG. 2 is a diagram for explaining a mapping method of each physicalchannel;

FIG. 3 is a diagram for explaining a mapping method of a PDCCH;

FIG. 4 is a diagram for explaining a search space of the PDCCH;

FIG. 5 is a diagram illustrating how a time-frequency resource for atransmission signal of a macro base station and a pico base station isallocated;

FIG. 6 is a diagram for explaining how a frequency resource for atransmission signal of each of the base stations is allocated;

FIG. 7 is a diagram illustrating a configuration of a wirelesscommunication system according to a first embodiment;

FIG. 8 is a diagram illustrating a configuration of a mobile stationaccording to the first embodiment;

FIG. 9 is a diagram illustrating an operation of the wirelesscommunication system according to the first embodiment;

FIG. 10 is a diagram for explaining a PDCCH multiplexing methodaccording to the first embodiment;

FIG. 11 is a diagram for explaining a scheduling algorithm in a picobase station according to the first embodiment;

FIG. 12 is a diagram illustrating a configuration of a wirelesscommunication system according to a fourth embodiment;

FIG. 13 is a diagram illustrating an operation of the wirelesscommunication system according to the fourth embodiment;

FIG. 14 is a diagram illustrating an example of interference reducingsubframe information according to the fourth embodiment;

FIG. 15 is a diagram for explaining a scheduling algorithm in a picobase station according to the fourth embodiment;

FIG. 16 is a diagram illustrating a configuration of a wirelesscommunication system according to a fifth embodiment; and

FIG. 17 is a diagram illustrating an operation of the wirelesscommunication system according to the fifth embodiment.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of a wireless communication system disclosed inthe present application will be explained in detail below with referenceto accompanying drawings. The present invention is not limited to theembodiments below.

First, a technology as a basis of the wireless communication systemdisclosed in the present application will be explained with reference toFIG. 1 to FIG. 6. FIG. 1 is a diagram illustrating an example of aheterogeneous network. As illustrated in FIG. 1, when a macrocell and apicocell are operated in a mixed manner, a downlink desired signal SG1from a pico base station to a mobile station connected to the pico basestation is greatly influenced by an interference signal SG2 from a macrobase station. Therefore, the communication quality is greatly reduced.

Before explanation of an influence of the inter-cell interference oneach physical channel, a configuration of each physical channel and amethod of mapping to a time-frequency resource will be explained belowwith reference to FIG. 2. FIG. 2 is a diagram for explaining a mappingmethod of each physical channel. As illustrated in FIG. 2, a subframewith the length of 1 ms is constituted of 14 OFDM symbols in the timedomain, and a control channel is mapped to first n (=1 to 3) OFDMsymbols. Examples of the control channel include a PCFICH (PhysicalControl Format Indicator CHannel), a PHICH (Physical Hybrid ARQIndicator CHannel), and a PDCCH (Physical Downlink Control CHannel).

The value of n is defined as control information called a CFI (ControlFormat Indicator). Shared channels PDSCH (Physical Downlink SharedCHannel) used to transmit user data or the like are mapped to theremaining OFDM symbols. In the frequency domain, an RB (Resource Block)serving as a unit for allocating a frequency resource is constituted of12 subcarriers, and shared channels for each user are frequency-divisionmultiplexed in an RB unit. A cell-specific reference signal(Cell-specific RS (Reference Signal)) used for channel estimation or thelike is sparsely mapped in the time and frequency domains. As a minimumunit of the time-frequency resource, an RE (Resource Element) that is aregion bounded by one OFDM symbol and one subcarrier is defined. As amapping unit of the control channel, an REG (Resource Element Group)constituted of four consecutive REs except for the RS in the frequencydomain is defined.

Of all the physical channels described above, in particular, a mappingmethod of the control channel will be explained in detail below. ThePCFICH is a physical channel used to transmit a CFI. Four REGs for thePCFICH are mapped so as to be distributed at approximately equalintervals within a system bandwidth with a starting point at theposition of a subcarrier dependent on a cell ID in the first OFDM symbolin the subframe.

The PHICH is a physical channel used to transmit ACK/NACK information onan uplink shared channel. The number of PHICH groups is determineddepending on a parameter Ng notified by a higher-level layer, and threeREGs are used for each of the PHICH groups. The three REGs are mapped soas to be distributed at approximately equal intervals within a systembandwidth with a starting point at the position of the subcarrierdependent on the cell ID among REGs to which the PCFICH is not mapped.

The PDCCH is a physical channel used to transmit notificationinformation and scheduling information on user data. FIG. 3 is a diagramfor explaining a method of mapping a PDCCH. A CCE (Control ChannelElement) is defined as a resource unit used by each PDCCH. The CCE isconstituted of nine REGs (=36 REs). An aggregation level (hereinafter,described as an “AL”) is a parameter corresponding to the number of CCEsused by the PDCCH, that is, a spreading factor. The AL is set to any of{1, 2, 4, 8} by a base station according to the state of a wirelesschannel or the like. While details will be explained later, each PDCCHis multiplexed by being added with appropriate offset, and is modulatedby QPSK (Quadrature Phase Shift Keying). Each PDCCH is interleaved inunits of four modulation symbols, and thereafter mapped to an REG towhich the PCFICH or the PHICH is not mapped.

Incidentally, the mobile station is not notified of a PDCCH multiplexingposition by the base station. Therefore, the mobile station searches forcandidates for possible multiplexing positions when decoding the PDCCH,and attempts to decode each receiving signal. To limit the number oftimes of decoding to the extent that the mobile station can process, aconcept of a search space (hereinafter, described as an “SS”) has beenintroduced. Therefore, the base station multiplexes the PDCCH at anarbitrary location within the limited search space, and the mobilestation searches through only the search space for an attempt ofdecoding.

FIG. 4 is a diagram for explaining a search space of the PDCCH. FIG. 4illustrates an example of search spaces in a certain subframe when 33CCEs are available. A common search space (Common Search Space) providedfor a PDCCH that transmits the scheduling information of thenotification information is always fixed to the first 16 CCEs. The headposition of a search space specific to the mobile station (UE (UserEquipment) Specific Search Space) provided for the PDCCH that transmitsthe scheduling information on user data differs for each of the mobilestations, the ALs, and the subframes. The head position is determined bya hash function. The number of available CCEs may change depending on asystem bandwidth, an antenna configuration, a CFI, or Ng.

An influence of the inter-cell interference on each of the physicalchannels will be explained below. FIG. 5 is a diagram illustrating how atime-frequency resource for a transmission signal of the macro basestation and the pico base station is allocated. In a system in which atransmission timing of a subframe is synchronized between cells, theinter-cell interference may occur between the shared channels andbetween the control channels. In LTE (Release-8), an FFR (FractionalFrequency Reuse) technology is applicable in order to reduce theinter-cell interference. For example, regarding the shared channels,specific RBs are allocated to the shared channels of a mobile station ata cell boundary in the pico base station. Meanwhile, in the macro basestation, the shared channels are not transmitted by the RBs ortransmitted with low transmission power so that the inter-cellinterference can be reduced.

However, in the technology described above, it is difficult to apply theFFR to the control channels because the control channels are arranged ina distributed manner in the whole system bandwidth. As a method forreducing the inter-cell interference in the control channels, there is amethod as illustrated in FIG. 6. FIG. 6 is a diagram for explaining howa frequency resource for a transmission signal of each base station isallocated. As illustrated in FIG. 6, a transmission timing of the picobase station is first shifted in an OFDM symbol unit with respect to themacro base station. Subsequently, shared channels of the macro basestation overlapping control channels of the pico base station areoverwritten (muting) with null symbols with transmission power of 0(zero). Therefore, the interference from the macrocell to the controlchannels of the picocell can be reduced. However, as described above,the reception characteristics of the shared channels of the macrocellare degraded.

First Embodiment

A configuration of a wireless communication system according to anembodiment disclosed in the present application will be explained below.FIG. 7 is a diagram illustrating a configuration of a wirelesscommunication system according to a first embodiment. As illustrated inFIG. 7, a wireless communication system 1 includes a pico base station100 and a macro base station 200. The pico base station 100 includes acontrol unit 100 a and a communicating unit 100 b. The control unit 100a includes an interference reducing CCE setting unit 101, a schedulerunit 102, a radio resource control unit 103, an inter-cell interferencedetermining unit 104, an uplink control signal demodulating unit 106,and a data signal generating unit 107. The control unit 100 a alsoincludes a control signal generating unit 108, a reference signalgenerating unit 109, a channel multiplexing unit 110, an IFFT (InversedFast Fourier Transform) unit 111, and a transmission timing control unit112. The communicating unit 100 b includes a reception RF unit 105 and atransmission RF (Radio Frequency) unit 113. All of the components areconnected to one another so as to be able to input and output a signalor data unidirectionally or bidirectionally. The control unit 100 a isphysically constructed of a digital circuit, a DSP (Digital SignalProcessor), a CPU (Central Processing Unit), or the like, and thecommunicating unit 100 b is physically constructed of an analog circuitincluding an amplifier and a filter, or the like.

The interference reducing CCE setting unit 101 sets an interferencereducing CCE based on a wireless parameter of the picocell, and notifiesthe scheduler unit 102 of the interference reducing CCE.

The scheduler unit 102 differs from a normal scheduler unit 204 (of themacro base station 200) in that the scheduler unit 102 allocates the CCEto a PDCCH for an interfered pico UE (User Equipment or a mobilestation) based on the interference reducing CCE. The scheduler unit 102determines allocation of a frequency resource to a data signal for eachof mobile stations, determines an MCS (Modulation and Coding Scheme),and determines the number of information bits or the like, based onchannel quality information (CQI (Channel Quality Indicator)) notifiedby each of the mobile stations. The scheduler unit 102 also determinesthe CFI according to the number of the mobile stations, and allocates anavailable CCE to a PDCCH for each of the mobile stations.

The radio resource control unit 103 notifies a radio resource controlunit 205 of the macro base station 200 of a muting request signal and awireless parameter (the number of antennas, a CFI, a cell ID, or Ng)needed to specify the CCE of the picocell. The notification is performedvia a wired interface. The radio resource control unit 103 receivesinformation on the amount of time shift to be described later from theradio resource control unit 205. The radio resource control unit 103controls handover based on information on the received power (RSRP(Reference Signal Received Power)) of each cell notified by each of themobile stations.

The inter-cell interference determining unit 104 estimates the state ofthe inter-cell interference in each of the mobile stations based on theinformation on the RSRP of each cell notified by each of the mobilestations, and determines whether to request application of muting (anoverwriting process with a null symbol). The determination result istransferred, as a muting request signal, to the radio resource controlunit 103.

The transmission timing control unit 112 shifts a transmission timing ofa downlink signal in an OFDM symbol unit based on the information on theamount of time shift.

Similarly, the macro base station 200 includes a control unit 200 a anda communicating unit 200 b. The control unit 200 a includes a mutingprocessing unit 201, an interference reducing CCE setting unit 202, amuting control unit 203, a scheduler unit 204, a radio resource controlunit 205, and an uplink control signal demodulating unit 207. Thecontrol unit 200 a also includes a reference signal generating unit 208,a control signal generating unit 209, a data signal generating unit 210,a channel multiplexing unit 211, and an IFFT unit 212. The communicatingunit 200 b includes a reception RF unit 206 and a transmission RF unit213. All of the components are connected to one another so as to be ableto input and output a signal or data unidirectionally orbidirectionally. The control unit 200 a is physically constructed of adigital circuit, a DSP, a CPU, or the like, and the communicating unit200 b is physically constructed of an analog circuit including anamplifier and a filter, or the like.

The muting processing unit 201 performs a muting process based on mutingcontrol information transferred by the muting control unit 203.

The interference reducing CCE setting unit 202 sets an interferencereducing CCE based on a wireless parameter of the picocell, and notifiesthe muting control unit 203 of the interference reducing CCE.

The muting control unit 203 determines execution of muting based on themuting request, and sets, as a muting region, a PDSCH RE of a macrocellcorresponding to the interference reducing CCE of the picocell.Thereafter, the muting control unit 203 notifies the muting processingunit 201 of muting region information. The muting control unit 203determines the amount of time shift of the picocell, and notifies theradio resource control unit 205 of the amount of time shift.

The scheduler unit 204 determines allocation of a frequency resource toa data signal for each of the mobile stations, determines an MCS(Modulation and Coding Scheme), and determines the number of informationbits or the like, based on the CQI notified by each of the mobilestations. The scheduler unit 204 also determines the CFI according tothe number of the mobile stations, and allocates an available CCE to aPDCCH for each of the mobile stations.

The radio resource control unit 205 notifies the radio resource controlunit 103 of the pico base station 100 of the information on the amountof time shift via a wired interface. The radio resource control unit 205receives the muting request signal and the wireless parameter (thenumber of antennas, a CFI, a cell ID, or Ng) of the picocell from theradio resource control unit 103. The radio resource control unit 205controls handover based on the information on RSRP of each cell notifiedby each of the mobile stations.

As other components, the pico base station 100 and the macro basestation 200 include a plurality of components that perform processingwith common contents. The reception RF units 105 and 206 convert a radiofrequency of an uplink received signal into a baseband, and performquadrature demodulation and A/D (Analog-to-Digital) conversion. Thereception RF units 105 and 206 include antennas A1 and A3, respectively,and receive uplink signals. The uplink control signal demodulating units106 and 207 demodulate uplink control signals and restore the CQI andthe RSRP, which are the control information, of each cell. The datasignal generating units 107 and 210 generate data signals based oninformation on the resource allocation, the MCS, or the like. Thecontrol signal generating units 108 and 209 generate control signalsbased on control information containing the resource allocationinformation or the like. The reference signal generating units 109 and208 generate reference signals. The channel multiplexing units 110 and211 multiplex frequencies of the respective physical channels. The IFFTunits 111 and 212 perform inverse Fourier transform (IFFT) and adds a CP(Cyclic Prefix). The transmission RF units 113 and 213 perform D/Aconversion, quadrature modulation, and conversion from a baseband into aradio frequency, and transmit downlink signals with amplified power. Thetransmission RF units 113 and 213 include antennas A2 and A4,respectively, and transmit downlink signals.

A configuration of a mobile station 10 will be explained below. FIG. 8is a diagram illustrating a configuration of the mobile stationaccording to the first embodiment. The mobile station 10 includes acontrol unit 10 a and a communicating unit 10 b. The control unit 10 aincludes an FFT unit 12, a data signal demodulating unit 13, a controlsignal demodulating unit 14, a channel estimating unit 15, a CQIcalculating unit 16, an RSRP measuring unit 17, and an uplink controlsignal generating unit 18. The communicating unit 10 b includes areception RF unit 11 and a transmission RF unit 19. All of thecomponents are connected to one another so as to be able to input andoutput a signal or data unidirectionally or bidirectionally.

The reception RF unit 11 converts a radio frequency of a downlinkreceived signal into a baseband, and performs quadrature demodulationand A/D conversion. The reception RF unit 11 receives a downlink signalvia an antenna A5. The FFT (Fast Fourier Transform) unit 12 detects atiming to extract a received signal, removes a CP, and converts thedetection result into a received signal in a frequency domain by Fouriertransform (FFT), similarly to a typical OFDM system. The data signaldemodulating unit 13 demodulates a data signal extracted from thereceived signal and restores data information based on the resourceallocation information. The control signal demodulating unit 14demodulates a control signal extracted from the received signal andrestores the resource allocation information as the control information.The channel estimating unit 15 calculates cross correlation between areference signal extracted from the received signal and a replica of aknown reference signal, to thereby obtain a channel estimation value.The channel estimation is performed not only on a cell to which themobile station 10 is connected but also on surrounding cells. The CQIcalculating unit 16 calculates channel quality information (a CQI asdescribed above) by using the channel estimation value of the cell towhich the mobile station 10 is connected. The RSRP measuring unit 17measures the received power (the RSRP as described above) of thereference signal of each of the cells by using the channel estimationvalues of the cell to which the mobile station 10 is connected and thesurrounding cells. The uplink control signal generating unit 18generates an uplink control signal based on the control informationcontaining the CQI and the RSRP of each of the cells. The transmissionRF unit 19 performs D/A (Digital-to-Analog) conversion and quadraturemodulation, and thereafter converts a baseband into a radio frequencyand transmits an uplink signal with amplified power. The transmission RFunit 19 transmits an uplink signal via an antenna A6. The control unit10 a is physically constructed of a digital circuit, a DSP, a CPU, orthe like, and the communicating unit 10 b is physically constructed ofan analog circuit including an amplifier and a filter, or the like.

An operation will be explained below. In the first embodiment, a networkenvironment is assumed in which one picocell is mixed in a macrocell asillustrated in FIG. 1. FIG. 9 is a diagram illustrating an operation ofthe wireless communication system 1 according to the first embodiment.In the explanation below, a mobile station connected to the pico basestation 100 is described as a pico UE and a mobile station connected tothe macro base station 200 is described as a macro UE.

At S1, the pico UE measures the received power of an RS (ReferenceSignal) of each of the connected cell and the surrounding cells, andnotifies the pico base station 100 of the measurement result as theRSRP.

At S2, the pico base station 100 estimates the state of inter-cellinterference in each of pico UEs based on the information on the RSRP ofeach of the cells notified by each of the pico UEs, and determineswhether to issue a muting request based on the estimation result. Forexample, assuming that the RSRP of the picocell as “RSRP_S” and the RSRPof an adjacent cell as “RSRP_I”, a parameter α=RSRP_S/RSRP_I representsthe state of the inter-cell interference in the pico UE. Therefore, amobile station for which α becomes smaller than a predeterminedthreshold (for example, an SINR (Signal to Interference and Noise Ratio)at which the block error rate of the PDCCH becomes 1% when AL=8 with thegreatest resistance against interference is applied) is defined as aninterfered pico UE. The number of the interfered pico UEs is used as acriterion, and application of muting is requested when the criterionbecomes equal to or greater than a predetermined threshold (for example,1). The criterion is not limited to the number of the interfered picoUEs but may be a ratio of the number of the interfered pico UEs to thetotal number of the pico UEs.

At S3, when requesting the application of muting, the pico base station100 notifies the macro base station 200 of the muting request signal anda wireless parameter (the number of antennas, a CFI, a cell ID, or Ng)needed to recognize the CCE of the picocell.

At S4, the macro base station 200 determines execution of muting basedon the muting request signal, and sets a muting region. Specifically,the macro base station 200 calculates the number of CCEs allocatable inthe picocell based on the wireless parameter of the picocell.Subsequently, the macro base station 200 defines a specific CCE (forexample a common SS) of the picocell as an interference reducing CCE,and sets a corresponding PDSCH RE of the macrocell as the muting region.The macro base station 200 determines the amount of time shift in thepicocell. The amount of time shift is determined as, for example, theCFI of the macrocell or the upper-limit value of the CFI, which is 3.

At S5, the macro base station 200 notifies the pico base station 100 ofthe amount of time shift determined at S4.

At S6, the pico base station 100 changes a transmission timing based onthe notified amount of time shift, and sets a CFI update cycle to avalue of a longer cycle. This is because, because the muting region ofthe macrocell is determined based on the CFI of the picocell, if the CFIis frequently changed, recognition of the CFI becomes inconsistentbetween the macrocell and the picocell. The pico base station 100 alsosets a specific CCE (for example, a common SS) as the interferencereducing CCE based on a rule shared with the macro base station 200.

FIG. 10 is a diagram for explaining a PDCCH multiplexing methodaccording to the first embodiment. At S7, as illustrated in FIG. 10, thepico base station 100 employs an interfered pico UE as a schedulingobject only when a UE-specific SS overlaps an interference reducing CCE,and allocates a part of the CCE in the overlapping region to the PDCCHfor the interfered pico UE.

FIG. 11 is a diagram for explaining a scheduling algorithm executed bythe pico base station 100 at S7. First, the pico base station 100selects a candidate UE for scheduling (S11), and determines whether itis possible to ensure a resource for a data signal (S12). As a result ofthe determination, when it is possible to ensure the resource (YES atS12), the pico base station 100 selects an AL (aggregation level) of thePDCCH (S13), and determines whether the UE selected at S11 is aninterfered pico UE (S14).

As a result of the determination at S14, when the UE selected at S11 isnot the interfered pico UE (NO at S14), the pico base station 100determines whether it is possible to ensure a resource for the PDCCH inthe SS with the AL selected at S13 (S15). As a result of thedetermination, when it is possible to ensure the resource (YES at S15),the pico base station 100 ensures the resource for the data signal andthe resource for the PDCCH (S16). Thereafter, the pico base station 100searches for other UEs to be candidates for the scheduling (S17). Whenthere is no other candidate UE (NO at S17), the scheduling process isfinished.

As a result of the determination at S14, when the UE selected at S11 isan interfered pico UE (YES at S14), the pico base station 100 determineswhether it is possible to ensure a resource for a PDCCH that overlaps aninterference reducing CCE in the SS with the AL selected at S13 (S18).As a result of the determination, when it is possible to ensure theresource (YES at S18), the pico base station 100 performs the process atS16 as described above. On the other hand, when it is not possible toensure the resource at S18 (NO at S18), the process returns to S11 andthe subsequent processes are performed again.

When it is determined that it is not possible to ensure the resource atS12 and S15 described above (NO at S12 and NO at S15), the processreturns to S11 and the subsequent processes are performed againsimilarly to the above.

A series of the processes at S11 to S18 described above is repeateduntil no candidate UE for a scheduling object remains (YES at S17), andis finished when the scheduling process on all of the candidate UEs iscompleted.

Referring back to FIG. 9, at S8, the pico base station 100 transmits aPDCCH for the interfered pico UE by using the interference reducing CCE.

At S9, the macro base station 200 transmits a PDSCH for the macro UE. Atthe time of the transmission, the macro base station 200 performs mutingon a muting region corresponding to the interference reducing CCE. Withthe muting, interference from the macrocell to the PDCCH for theinterfered pico UE in the picocell is reduced.

The explanation of the operation with reference to the flowchart iscomplete.

As described above, the wireless communication system 1 of the firstembodiment controls a transmission timing in each cell so that the PDCCHof the picocell and the PDSCH of the macrocell temporally overlap eachother. The wireless communication system 1 includes the pico basestation 100, the macro base station 200, and the pico mobile station 10.The pico base station 100 includes the control unit 100 a and thecommunicating unit 100 b. The control unit 100 a notifies the macro basestation 200 of information used to specify a resource of the PDCCH,where the resource corresponds to a predetermined resource unit. Thecommunicating unit 100 b transmits a control signal to the pico mobilestation 10 by using a first resource of the PDCCH of the picocell, wherethe first resource corresponds to at least a part of the predeterminedresource unit and serves as a decoding object of the pico mobile station10. The macro base station 200 includes the communicating unit 200 bthat transmits a null symbol by using a second resource of the PDSCH ofthe macrocell, where the second resource corresponds to thepredetermined resource unit. The pico mobile station 10 includes thecommunicating unit 10 b that receives the control signal transmitted bythe pico base station 100 via the first resource, and that receives anull symbol transmitted by the macro base station 200 via the secondresource. Incidentally, the predetermined resource unit is one or moreCCEs, and is, for example, an interference reducing CCE. The firstresource is a search space specific to the pico mobile station 10, andis, for example, a resource element to which a CCE in a region where theinterference reducing CCE and a user-specific search space overlaps eachother is mapped in a time-frequency domain. The second resource is aresource element to which the interference reducing CCE is mapped in atime-frequency domain. Therefore, the wireless communication system 1can reduce interference with the PDCCH while maintaining the receptioncharacteristics of the PDSCH.

Second Embodiment

In a second embodiment, an example will be explained in which a TPC(Transmission Power Control) technology is applied to the wirelesscommunication system of the first embodiment. Specifically, thescheduler unit 102 of the pico base station 100 of the first embodimentemploys an interfered pico UE as a scheduling object only when theUE-specific SS overlaps the interference reducing CCE. However, becausethe head position of the UE-specific SS differs for each of the mobilestations, the ALs, and the subframes, the SS for an optimal AL for awireless channel state does not always overlap the interference reducingCCE.

Therefore, if it is possible to select an AL other than the optimal ALfor the wireless channel state, it becomes possible to improve theprobability that the UE-specific SS overlaps the interference reducingCCE. Consequently, it becomes possible to increase opportunities toperform scheduling on the interfered pico UE, so that the throughput maybe improved. However, it is concerned that a block error of the PDDCHfrequently occurs if an AL other than the optimal AL for the wirelesschannel state is simply applied.

Therefore, a wireless communication system according to the secondembodiment concurrently performs transmission power control (TPC) toavoid a concerning situation as described above. For example, assumingthat the optimal AL for the wireless channel state is AL_(opt) and theAL actually applied is AL_(sel), the scheduler unit 102 multiplies thetransmission power per RE by (AL_(opt)/AL_(sel)). Therefore, thetransmission power per PDCCH resource becomes equal to the transmissionpower obtained when AL=AL_(opt) is applied. As a result, the block errorof the PDCCH can be suppressed.

As described above, in the wireless communication system 1 of the secondembodiment, the control unit 100 a of the pico base station 100provisionally selects an aggregation level to be applied to the PDCCHfor the mobile station 10 connected to the picocell based on thewireless channel state of the mobile station 10. When a UE-specific SScorresponding to the provisional aggregation level of the mobile station10 does not overlap the interference reducing CCE, the communicatingunit 100 b of the pico base station 100 selects an aggregation level,for which the UE-specific SS overlaps the interference reducing CCE andwhich is equal to or greater than the provisional value, and performstransmission. Alternatively, the communicating unit 100 b selects anaggregation level, for which the UE-specific SS overlaps theinterference reducing CCE and which is smaller than the provisionalvalue, and performs transmission with transmission power adjusted to beequal to or greater than a predetermined value. Consequently, it ispossible to suppress the block error of the PDCCH and improves theprobability that the UE-specific SS overlaps the interference reducingCCE. As a result, it is possible to increase opportunities to performscheduling on the interfered pico UE, so that the system throughput canbe improved.

Third Embodiment

In a third embodiment, an example will be explained in which a bulkmuting technology for a plurality of pico base stations is applied tothe wireless communication system of the first embodiment. In the thirdembodiment, a network environment is assumed in which a plurality ofpicocells are mixed in a macrocell, which is different from the firstembodiment.

In the third embodiment, it is concerned how the muting control unit 203of the macro base station 200 determines whether muting is actuallyapplied based on the muting request signal notified by a plurality ofthe pico base stations. A criterion for the determination may be basedon whether the number of pico base stations that have requestedapplication of muting exceeds a predetermined threshold.

As a first example, there is a case in which an emphasis is placed onreduction of interference from the macro base station 200 to the controlchannel of the pico base station 100 as a design principle of the mobilecommunication system. In this case, it is sufficient that the mutingcontrol unit 203 actually applies muting when one or more pico basestations have requested application of muting.

As a second example, there is a trade-off relation of the transmissionefficiencies of the pico base station 100 and the macro base station200. Specifically, while the reception characteristics of the pico basestation 100 improves with an increase in the amount of muting, thereception characteristics of the macro base station 200 is sacrificed.In view of the above circumstances, there is a case in which an emphasisis placed on balancing between the characteristics. In this case, it issufficient that the muting control unit 203 actually applies muting whenover half of the pico base stations have requested application ofmuting.

A point that needs to be considered in the third embodiment is how toset interference reducing CCEs in a plurality of the pico base stationsand how to set a muting region in the macro base station 200. Forexample, the muting control unit 203 defines a specific CCE (forexample, a common SS) of each of the picocells as an interferencereducing CCE similarly to the first embodiment. A correspondence betweenthe CCE and the position of an RE to which the CCE is mapped depends onthe number of transmission antennas, a CFI, a cell ID, and Ng of each ofthe picocells. Therefore, the correspondence is not always the samebetween the picocells. Therefore, it is sufficient that the mutingcontrol unit 203 sets a muting region so as to include all of the REs towhich the interference reducing CCEs of all of the picocells are mapped.

As described above, in the wireless communication system 1 according tothe third embodiment, the macro base station 200 includes the controlunit 200 a and the communicating unit 200 b. The control unit 200 acalculates, for each of the picocells, an RE to which the interferencereducing CCE of the picocell is mapped based on the wireless parameterof each of the picocells. The communicating unit 200 b transmits a nullsymbol in each of the REs calculated by the control unit 200 a by usingthe same subframe. Therefore, in the network environment in which aplurality of the picocells are mixed in the macrocell, a bulk mutingtechnology for a plurality of the pico base stations can be applied tothe wireless communication system of the first embodiment. As a result,it is possible to reduce interference from a single macrocell to thePDCCHs of the picocells.

In the wireless communication system 1 according to the thirdembodiment, the control unit 100 a of the pico base station 100 notifiesthe macro base station 200 of a wireless parameter of the picocell asinformation needed to specify a resource to which the interferencereducing CCE is mapped. The control unit 200 a of the macro base station200 specifies the resource to which the interference reducing CCE of thepicocell is mapped based on the information on the wireless parameternotified by the pico base station 100. Therefore, the macro base station200 can accurately recognize the position of an RE to which the PDCCH ofthe picocell is highly likely to be mapped.

Fourth Embodiment

In a fourth embodiment, an example will be explained in which atime-division muting technology for a plurality of pico base stations isapplied to the wireless communication system of the first embodiment. Inthe fourth embodiment, a network environment is assumed in which aplurality of picocells are mixed in a macrocell, which is different fromthe first embodiment.

In the third embodiment, the muting control unit 203 sets a mutingregion of the macrocell so as to include all of the REs to which theinterference reducing CCEs of all of the picocells are mapped.Therefore, it becomes possible to collectively control interference withthe interfered pico UEs in a plurality of the picocells. However, aneeded muting region tends to be greater. Therefore, in the fourthembodiment, the wireless communication system controls interference withthe interfered pico UEs in a plurality of picocells while maintaining asmall needed muting region. To control the interference as describedabove, the muting control unit of the macro base station performs mutingspecifically on an individual picocell for each subframe.

A configuration of the wireless communication system according to thefourth embodiment will be explained below. FIG. 12 is a diagramillustrating a configuration of the wireless communication systemaccording to the fourth embodiment. As illustrated in FIG. 12, awireless communication system 2 includes a pico base station 300 and amacro base station 400. The pico base station 300 includes a controlunit 300 a and a communicating unit 300 b. The control unit 300 aincludes an interference reducing CCE setting unit 301, a scheduler unit302, a radio resource control unit 303, an inter-cell interferencedetermining unit 304, an uplink control signal demodulating unit 306,and a data signal generating unit 307. The control unit 300 a alsoincludes a control signal generating unit 308, a reference signalgenerating unit 309, a channel multiplexing unit 310, an IFFT unit 311,and a transmission timing control unit 312. The communicating unit 300 bincludes a reception RF unit 305 and a transmission RF unit 313. All ofthe components are connected to one another so as to be able to inputand output a signal or data unidirectionally or bidirectionally.

Similarly, the macro base station 400 includes a control unit 400 a anda communicating unit 400 b. The control unit 400 a includes a mutingprocessing unit 401, an interference reducing CCE setting unit 402, amuting control unit 403, a scheduler unit 404, a radio resource controlunit 405, and an uplink control signal demodulating unit 407. Thecontrol unit 400 a also includes a reference signal generating unit 408,a control signal generating unit 409, a data signal generating unit 410,a channel multiplexing unit 411, and an IFFT unit 412. The communicatingunit 400 b includes a reception RF unit 406 and a transmission RF unit413. All of the components are connected to one another so as to be ableto input and output a signal or data unidirectionally orbidirectionally.

The wireless communication system 2 has the same configuration as thatof the wireless communication system 1 of the first embodiment.Therefore, the same components are denoted by reference symbols with thesame trailing numerals, and detailed explanation thereof will beomitted.

Specifically, the pico base station 300 and the macro base station 400of the fourth embodiment are components corresponding to the pico basestation 100 and the macro base station 200 of the first embodiment,respectively. The control unit 300 a and the communicating unit 300 b ofthe pico base station 300 correspond to the control unit 100 a and thecommunicating unit 100 b of the pico base station 100, respectively.Similarly, the control unit 400 a and the communicating unit 400 b ofthe macro base station 400 correspond to the control unit 200 a and thecommunicating unit 200 b of the macro base station 200, respectively.

The interference reducing CCE setting unit 301, the scheduler unit 302,and the radio resource control unit 303 of the pico base station 300correspond to the interference reducing CCE setting unit 101, thescheduler unit 102, and the radio resource control unit 103 of the picobase station 100, respectively. The inter-cell interference determiningunit 304, the reception RF unit 305, the uplink control signaldemodulating unit 306, and the data signal generating unit 307correspond to the inter-cell interference determining unit 104, thereception RF unit 105, the uplink control signal demodulating unit 106,and the data signal generating unit 107, respectively. The controlsignal generating unit 308, the reference signal generating unit 309,and the channel multiplexing unit 310 correspond to the control signalgenerating unit 108, the reference signal generating unit 109, and thechannel multiplexing unit 110, respectively. The IFFT unit 311, thetransmission timing control unit 312, and the transmission RF unit 313correspond to the IFFT unit 111, the transmission timing control unit112, and the transmission RF unit 113, respectively.

Similarly, the muting processing unit 401 and the interference reducingCCE setting unit 402 of the macro base station 400 correspond to themuting processing unit 201 and the interference reducing CCE settingunit 202 of the macro base station 200, respectively. The muting controlunit 403, the scheduler unit 404, and the radio resource control unit405 correspond to the muting control unit 203, the scheduler unit 204,and the radio resource control unit 205, respectively. The reception RFunit 406, the uplink control signal demodulating unit 407, and thereference signal generating unit 408 correspond to the reception RF unit206, the uplink control signal demodulating unit 207, and the referencesignal generating unit 208, respectively. The control signal generatingunit 409, the data signal generating unit 410, and the channelmultiplexing unit 411 correspond to the control signal generating unit209, the data signal generating unit 210, and the channel multiplexingunit 211, respectively. The IFFT unit 412 and the transmission RF unit413 correspond to the IFFT unit 212 and the transmission RF unit 213,respectively.

The configuration of the mobile station is the same as that of the firstembodiment, and therefore, explanation thereof will be omitted.

A main difference between the fourth embodiment and the first embodimentwill be explained below. The inter-cell interference determining unit304 of the pico base station 300 estimates a state of inter-cellinterference in each of the mobile stations based on information on theRSRP of each cell notified by each of the mobile stations. Theinter-cell interference determining unit 304 determines whether torequest application of muting based on the estimation result, andgenerates information on the number of interfered UEs. The inter-cellinterference determining unit 304 transfers the information on thenumber of the interfered UEs to the radio resource control unit 303. Theradio resource control unit 303 of the pico base station 300 notifiesthe radio resource control unit 405 of the macro base station 400 ofinformation on the number of the interfered UEs and a wireless parameter(the number of antennas, a CFI, a cell ID, or Ng) needed to recognizethe CCE of the picocell. The radio resource control unit 303 receivesinformation on the amount of time shift and interference reducingsubframe information for each of the picocells from the macro basestation 400. The scheduler unit 302 of the pico base station 300allocates a CCE to a PDCCH for the interfered pico UE based on theinterference reducing subframe information for each of the picocells andthe interference reducing CCE.

Meanwhile, the muting control unit 403 of the macro base station 400generates interference reducing subframe information for each of thepicocells based on the information on the number of the interfered UEsfor each of the picocells. The muting control unit 403 sets a PDSCH REof a macrocell corresponding to the interference reducing CCE of each ofthe picocells as the muting region for each of the picocells. The mutingcontrol unit 403 notifies the muting processing unit 401 of the mutingregion information for the picocell, for which an interference reducingsubframe is set, for each subframe.

The muting control unit 403 determines the amount of time shift commonto all of the picocells, and notifies the radio resource control unit405 of the amount of time shift together with the interference reducingsubframe information for each of the picocells.

The radio resource control unit 405 of the macro base station 400notifies the radio resource control unit 303 of each of the pico basestations 300 of the information on the amount of time shift and theinterference reducing subframe information for each of the picocells,via a wired interface. The radio resource control unit 405 receives theinformation on the number of interfered UEs and the wireless parameter(the number of antennas, a CFI, a cell ID, or Ng) of each of thepicocells from the pico base stations 300. The radio resource controlunit 405 also transmits and receives various types of data and signalsto and from a pico base station 500.

An operation will be explained below. In the fourth embodiment, anetwork environment is assumed in which two picocells are mixed in amacrocell. FIG. 13 is a diagram illustrating an operation of thewireless communication system 2 according to the fourth embodiment. Inthe explanation below, a mobile station connected to the pico basestation 300 is described as a pico UE 10, a mobile station connected tothe pico base station 500 is described as a pico UE 20, and a mobilestation connected to the macro base station 400 is described as a macroUE. Furthermore, a picocell formed by the pico base station 300 isdescribed as a picocell C1, and a picocell formed by the pico basestation 500 is described as a picocell C2.

At S21, the pico UE 10 measures the received power of the RS of each ofthe connected cell and the surrounding cells, and notifies the pico basestation 300 of the measurement result as the RSRP. The pico UE 20performs the same process as S21, and notifies the pico base station 500of the RSRP of each of the cells (S22).

At S23, the pico base station 300 estimates the state of the inter-cellinterference of each of the pico UEs based on the information on theRSRP of each of the cells notified by each of the pico UEs, anddetermines whether to request muting based on the estimation result. Thepico base station 500 performs the same estimation process and thedetermination process (S24). The detailed processing contents are thesame as those of the process at S2 in FIG. 9 of the first embodiment,and therefore, detailed explanation thereof will be omitted.

At S25, when requesting application of the muting, the pico base station300 notifies the macro base station 400 of information on the number ofinterfered UEs and a wireless parameter (the number of antennas, a CFI,a cell ID, or Ng) needed to recognize the picocell C1. Incidentally, asthe information on the number of the interfered UEs, the number of theinterfered pico UEs is used that serves as a criterion for determiningwhether to request muting. The information on the number of theinterfered UEs is not limited to the above number, and may be a ratio ofthe number of the interfered pico UEs to the total number of the picoUEs. At S26, the pico base station 500 performs the same process as S25on the macro base station.

At S27, the macro base station 400 determines a method for applying themuting based on the information on the number of the interfered UEs andthe picocell information obtained from the pico base stations 300 and500. Specifically, the macro base station 400 defines an interferencereducing CCE for each of target picocells, and sets a correspondingPDSCH RE of the macrocell as a muting region. The macro base station 400also determines the interference reducing CCE for the target picocelland a subframe (interference reducing subframe) in which the mutingregion is set, based on the information on the number of the interferedUEs. For example, when values indicated by the information on the numberof the interfered UEs of the picocells C1 and C2 are p_mute1 andp_mute2, respectively, the macro base station 400 determines the ratioof the number of the interference reducing subframes of the picocells C1and C2 as (p_mute1:p_mute2). The macro base station 400 generatesinformation (interference reducing subframe information) indicating theposition of the interference reducing subframe for each of the picocellsC1 and C2 in the subframe based on the above ratio.

FIG. 14 is a diagram illustrating an example of the interferencereducing subframe information according to the fourth embodiment. FIG.14 illustrates the interference reducing subframe information defined bya 10-subframe cycle when (p_mute1:p_mute2)=(3:7). In FIG. 14, in thesubframe number for which “1” is assigned, an interference reducingsubframe for a corresponding picocell is set. The macro base station 400determines the amount of time shift common to the picocells C1 and C2.The amount of time shift is determined as, for example, the CFI of themacrocell or the upper-limit value of the CFI, which is 3.

Referring back to FIG. 13, at S28, the macro base station 400 notifiesthe pico base station 500 of the interference reducing subframeinformation and the amount of time shift determined at S27. The piecesof the information are also sent from the macro base station 400 to thepico base station 300 (S29).

At S30, the pico base station 300 changes a transmission timing based onthe amount of time shift notified by the macro base station 400 and setsthe CFI update cycle to a value of a longer cycle. The pico base station300 sets a specific CCE (for example, a common SS) as the interferencereducing CCE of the picocell C1 based on a rule shared with the macrobase station 400.

At S31, only when the interference reducing subframe for a picocell isset with respect to the interfered pico UE and the UE-specific SSoverlaps the interference reducing CCE of the picocell C1, the pico basestation 300 applies the interfered pico UE as a scheduling object. Thepico base station 300 allocates a part of the CCE in the overlappingregion to the PDCCH for the interfered pico UE.

A scheduling algorithm according to the fourth embodiment will beexplained below. FIG. 15 is a diagram for explaining the schedulingalgorithm of the pico base stations 300 and 500 according to the fourthembodiment. FIG. 15 is the same as FIG. 11 except for a determinationprocess that is newly added at Step S49. Therefore, detailed explanationof FIG. 15 will be omitted. Steps S41 to S48 in FIG. 15 correspond toSteps S11 to S18 in FIG. 11, respectively.

In the first embodiment, it has been explained that the determination onwhether the selected candidate UE is an interfered pico UE (S14 in FIG.11) is performed, and thereafter determination on whether it is possibleto ensure a resource for the PDCCH is performed regardless of a resultof the former determination (S15 or S18). By contrast, in the fourthembodiment, when the UE selected at S41 is an interfered pico UE (YES atS44), each of the pico base stations 300 and 500 determines whether acorresponding subframe is an interference reducing subframe for acorresponding picocell (S49). As a result of the determination, when thesubframe is the interference reducing subframe (YES at S49), the processproceeds to S48. On the other hand, when the subframe is not theinterference reducing subframe (NO at S49), the process returns to S41and the processes from S41 are performed again. The processes to beperformed when the selected candidate UE is not the interfered pico UEat S44 (NO at S44) are the same as those of the first embodiment.

Referring back to FIG. 13, the pico base station 500 performs the sameprocesses as S30 and S31 described above, based on the interferencereducing subframe information and the amount of time shift notified atS28 (S32 and S33).

At S34, the pico base station 300 transmits a PDCCH for the interferedpico UE in the interference reducing subframe for the picocell C1 byusing the interference reducing CCE for the picocell C1. At the sametime, when the macro base station 400 transmits a PDSCH, muting isperformed on the muting region corresponding to the interferencereducing CCE for the picocell C1 (S35). Therefore, interference from themacrocell to the PDCCH for the interfered pico UE in the picocell C1 canbe reduced.

At S36, the pico base station 500 transmits a PDCCH for the interferedpico UE in the interference reducing subframe for the picocell C2 byusing the interference reducing CCE for the picocell C2. At the sametime, when the macro base station 400 transmits a PDSCH, muting isperformed on a muting region corresponding to the interference reducingCCE corresponding to the picocell C2 (S37). Therefore, interference fromthe macrocell to the PDCCH for the interfered pico UE in the picocell C2can be reduced.

In the fourth embodiment, the picocells C1 and C2 have been explained asrepresentative picocells. Meanwhile, the positions of REs to which thePDCCHs of picocells other than the representative picocells are mappedpartially overlap the muting region. Therefore, it is also possible topartially reduce interference with the PDCCH.

As described above, the wireless communication system 2 of the fourthembodiment includes a plurality of the pico base stations 300 and 500.The pico base station 300 includes the control unit 300 a and thecommunicating unit 300 b. The control unit 300 a notifies the macro basestation 400 of the information needed to specify a CCE to which theinterference reducing CCE is mapped and the information on the number ofinterfered UEs. A case is assumed in which UE-specific search spaces ofmobile stations connected to the picocells C1 and C2 overlap theinterference reducing CCE in the interference reducing subframes for thepicocells C1 and C2 indicated by the interference reducing subframeinformation notified by the macro base station 400. In this case, thecommunicating unit 300 b performs transmission by a PDCCH for the mobilestation by using the interference reducing CCE in the overlappingregion. The macro base station 400 includes the control unit 400 a andthe communicating unit 400 b. The control unit 400 a sets theinterference reducing subframe for each of the picocells C1 and C2 basedon the information on the number of the interfered UEs notified by aplurality of the pico base stations 300 and 500, and notifies each ofthe pico base stations 300 and 500 of the interference reducing subframeinformation. The communicating unit 400 b transmits a null symbol in aresource to which the interference reducing CCE of the picocell ismapped in the interference reducing subframe for each of the picocellsC1 and C2. Therefore, in the network environment in which a plurality ofthe picocells C1 and C2 is mixed in a macrocell, it is possible to applythe time-division muting technology to the wireless communication systemof the first embodiment. Therefore, interference from a single macrocellto the PDCCHs of the picocells C1 and C2 can be reduced. In addition,the reception characteristics of the PDSCH of the macrocell can bemaintained regardless of the number of the picocells.

In the wireless communication system 2, the control unit 300 a of thepico base station 300 notifies the macro base station 400 of informationbased on the number of the UEs in the wireless channel state of apredetermined quality or lower, as the information on the number of theinterfered UEs. The wireless channel state of the predetermined qualityor lower indicates, for example, a state in which a measured receptionlevel is equal to or lower than a predetermined value. Therefore, thecontrol unit 300 a can issue more appropriate information indicating howmany mobile stations are greatly interfered.

In the wireless communication system 2, the control unit 400 a of themacro base station 400 sets a ratio of the picocells C1 and C2 withrespect to the number of the interference reducing subframes based onthe ratio of the picocells C1 and C2 with respect to values indicated bythe information on the number of the interfered UEs. Therefore, it ispossible to prevent inequality of opportunity for transmission of thePDCCHs of the picocells C1 and C2 between the cells.

Furthermore, in the wireless communication system 2, the control unit400 a of the macro base station 400 sets the interference reducingsubframes of the picocells C1 and C2 so that the subframes do notoverlap each other. Therefore, the wireless communication system 2 canassuredly apply an optimal interference control process for each of thepicocells in each of the subframes.

Moreover, in the wireless communication system 2, the communicating unit400 b of the macro base station 400 transmits a null symbol to themobile station 10 of the pico base station 300 in response to a requestfrom a pico base station that has issued a request for transmission ofthe null symbol from among a plurality of the pico base stations 300 and500. Therefore, muting is performed only on a pico base station forwhich interference needs to be controlled. Therefore, compared with acase in which the muting is performed on all of the pico base stations,it is possible to reduce processing load on the macro base station 400due to control of the interference.

Fifth Embodiment

In a fifth embodiment, an example will be explained in which atechnology for adaptively controls the interference reducing CCE isapplied to the wireless communication system of the first embodiment.Specifically, in the first embodiment, the wireless communication system1 sets the interference reducing CCE in, for example, the common SSaccording to the rule common to the macro base station and the pico basestation. However, if the interference reducing CCE is fixedly set, when,for example, the number of the interfered pico UEs in the picocellincreases, it becomes difficult for the wireless communication system 1to ensure resources for the PDCCHs of the interfered pico UEs and thethroughput of the interfered pico UEs may be reduced. On the other hand,when the number of the interfered pico UEs is small, while the wirelesscommunication system 1 can easily ensure a resource for the PDCCHs ofthe interfered pico UEs, the number of the PDSCH REs muted in themacrocell becomes excessive. Therefore, a wireless communication systemaccording to a fifth embodiment controls the interference reducing CCEaccording to the communication status of the picocell.

A configuration of the wireless communication system according to thefifth embodiment will be explained below. FIG. 16 is a diagramillustrating the configuration of the wireless communication systemaccording to the fifth embodiment. As illustrated in FIG. 16, a wirelesscommunication system 3 includes a pico base station 600 and a macro basestation 700. The pico base station 600 includes a control unit 600 a anda communicating unit 600 b. The control unit 600 a includes a schedulerunit 602, a radio resource control unit 603, an inter-cell interferencedetermining unit 604, an uplink control signal demodulating unit 606,and a data signal generating unit 607. The control unit 600 a alsoincludes a control signal generating unit 608, a reference signalgenerating unit 609, a channel multiplexing unit 610, an IFFT unit 611,and a transmission timing control unit 612. The communicating unit 600 bincludes a reception RF unit 605 and a transmission RF unit 613. All ofthe components are connected to one another so as to be able to inputand output a signal or data unidirectionally or bidirectionally.

Similarly, the macro base station 700 includes a control unit 700 a anda communicating unit 700 b. The control unit 700 a includes a mutingprocessing unit 701, an interference reducing CCE setting unit 702, amuting control unit 703, a scheduler unit 704, a radio resource controlunit 705, and an uplink control signal demodulating unit 707. Thecontrol unit 700 a also includes a reference signal generating unit 708,a control signal generating unit 709, a data signal generating unit 710,a channel multiplexing unit 711, and an IFFT unit 712. The communicatingunit 700 b includes a reception RF unit 706 and a transmission RF unit713. All of the components are connected to one another so as to be ableto input and output a signal or data unidirectionally orbidirectionally.

The wireless communication system 3 has the same configuration as thatof the wireless communication system 1 of the first embodiment.Therefore, the same components are denoted by reference symbols with thesame trailing numerals, and explanation thereof will be omitted.

Specifically, the pico base station 600 and the macro base station 700of the fifth embodiment are components corresponding to the pico basestation 100 and the macro base station 200 of the first embodiment,respectively. The control unit 600 a and the communicating unit 600 b ofthe pico base station 600 correspond to the control unit 100 a and thecommunicating unit 100 b of the pico base station 100, respectively.Similarly, the control unit 700 a and the communicating unit 700 b ofthe macro base station 700 correspond to the control unit 200 a and thecommunicating unit 200 b of the macro base station 200, respectively.

The scheduler unit 602 and the radio resource control unit 603 of thepico base station 600 correspond to the scheduler unit 102 and the radioresource control unit 103 of the pico base station 100, respectively.The inter-cell interference determining unit 604, the reception RF unit605, the uplink control signal demodulating unit 606, and the datasignal generating unit 607 correspond to the inter-cell interferencedetermining unit 104, the reception RF unit 105, the uplink controlsignal demodulating unit 106, and the data signal generating unit 107,respectively. The control signal generating unit 608, the referencesignal generating unit 609, and the channel multiplexing unit 610correspond to the control signal generating unit 108, the referencesignal generating unit 109, and the channel multiplexing unit 110,respectively. Furthermore, the IFFT unit 611, the transmission timingcontrol unit 612, and the transmission RF unit 613 correspond to theIFFT unit 111, the transmission timing control unit 112, and thetransmission RF unit 113, respectively.

Similarly, the muting processing unit 701 and the interference reducingCCE setting unit 702 of the macro base station 700 correspond to themuting processing unit 201 and the interference reducing CCE settingunit 202 of the macro base station 200, respectively. The muting controlunit 703, the scheduler unit 704, and the radio resource control unit705 correspond to the muting control unit 203, the scheduler unit 204,and the radio resource control unit 205, respectively. The reception RFunit 706, the uplink control signal demodulating unit 707, and thereference signal generating unit 708 correspond to the reception RF unit206, the uplink control signal demodulating unit 207, and the referencesignal generating unit 208, respectively. The control signal generatingunit 709, the data signal generating unit 710, and the channelmultiplexing unit 711 correspond to the control signal generating unit209, the data signal generating unit 210, and the channel multiplexingunit 211, respectively. The IFFT unit 712 and the transmission RF unit713 correspond to the IFFT unit 212 and the transmission RF unit 213,respectively.

The configuration of the mobile station is the same as that of the firstembodiment, and therefore, explanation thereof will be omitted.

A main difference between the fifth embodiment and the first embodimentwill be explained below. The interference reducing CCE setting unit 702of the macro base station 700 adjusts the interference reducing CCEbased on information on the number of interfered UEs and picocellinformation, and notifies the muting control unit 703 and the radioresource control unit 705 of the adjustment result as interferencereducing CCE information. The radio resource control unit 705 of themacro base station 700 notifies the radio resource control unit 603 ofthe pico base station 600 of the amount of time shift and theinterference reducing CCE information. The radio resource control unit603 of the pico base station 600 notifies the scheduler unit 602 of theinterference reducing CCE information.

An operation will be explained below. FIG. 17 is a diagram illustratingan operation of the wireless communication system 3 according to thefifth embodiment. FIG. 17 is the same as FIG. 9 except for Steps S53 toS55. Therefore, detailed explanation of FIG. 17 will be omitted and adifference from the first embodiment will be explained below. Steps S51to S59 in FIG. 17 correspond to Step S1 to S9 illustrated in FIG. 9,respectively.

At S53, the pico base station 600 notifies the macro base station 700 ofthe picocell information (the number of antennas, a CFI, a cell ID, andNg) and information on the number of interfered UEs (mobile stations).The macro base station 700 adjusts the number of the interferencereducing CCEs based on the information on the number of the interferedUEs notified by the pico base station 600 (S54). Specifically, when thenumber of the interfered UEs indicated by the information on the numberof the interfered UEs or the ratio of the number of the interfered UEsis equal to or greater than a predetermined value, the macro basestation 700 increases the number of the interference reducing CCEs. Onthe other hand, when the number of the interfered UEs indicated by theinformation on the number of the interfered UEs or the ratio of thenumber of the interfered UEs is smaller than the predetermined value,the macro base station 700 decreases the number of the interferencereducing CCEs. At S55, the macro base station 700 notifies the pico basestation 600 of the information on the number of the interferencereducing CCEs adjusted at S54, as interference reducing CCE information,together with the amount of time shift.

The pico base station 600 sets an interference reducing CCE according tothe interference reducing CCE information notified by the macro basestation 700 at S55.

As described above, the wireless communication system 3 of the fifthembodiment includes the pico base station 600 and the macro base station700. The pico base station 600 includes the control unit 600 a. Thecontrol unit 600 a notifies the macro base station 700 of theinformation on the number of the interfered UEs, and sets theinterference reducing CCE according to the interference reducing CCEinformation notified by the macro base station 700. The macro basestation 700 includes the control unit 700 a. The control unit 700 aadjusts the number of the interference reducing CCEs based on theinformation on the number of the interfered UEs notified by the picobase station 600, and notifies the pico base station 600 of theinterference reducing CCE information based on the number of theinterference reducing CCEs. Therefore, even when the number of theinterfered pico UEs in the picocell changes according to time or place,the wireless communication system 3 can ensure adequate resources forthe PDCCHs of the interfered pico UEs in the picocell. Furthermore, theamount of the PDSCH REs to be muted in the macrocell can be reduced to arequisite minimum.

In the wireless communication system 3, the control unit 700 a of themacro base station 700 increases the number of the interference reducingCCEs when the number of the interfered UEs is equal to or greater than apredetermined value, and decreases the number of the interferencereducing CCEs when the number of the interfered UEs is smaller than thepredetermined value. Therefore, the number of the interference reducingCCEs is appropriately adjusted according to the communication status ofthe picocell. Consequently, it becomes possible to control theinterference reducing CCEs in just proportion for each of the pico basestations.

In the above embodiments, the wireless communication system disclosed inthe present application reduces interference between the macrocell andthe picocell. However, the wireless communication systems 1, 2, and 3are not limited to the above, and may be applied to a technology forreducing interference between a macrocell and a femtocell orinterference between a picocell and a femtocell.

According to one embodiment of the wireless communication systemdisclosed in the present application, it is possible to reduceinterference with a control channel while maintaining the receptioncharacteristics of a data channel.

All examples and conditional language provided herein are intended forthe pedagogical purposes of aiding the reader in understanding theinvention and the concepts contributed by the inventors to further theart, and are not to be construed as limitations to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although one or more embodiments of thepresent invention have been described in detail, it should be understoodthat the various changes, substitutions, and alterations could be madehereto without departing from the spirit and scope of the invention.

What is claimed is:
 1. A wireless communication system that controls atransmission timing in each of cells so that a control channel of afirst cell and a data channel of a second cell temporally overlap eachother, wherein the wireless communication system comprises a basestation of the first cell, a base station of the second cell, and amobile station of the first cell, and the base station of the first cellincludes: a processor configured to execute a first process including:notifying a base station of the second cell of information used tospecify a resource of the control channel of the first cell, theresource corresponding to a predetermined resource unit; andtransmitting a control signal to a mobile station of the first cell byusing a first resource of the control channel of the first cell, thefirst resource corresponding to at least a part of the predeterminedresource unit and serving as a decoding object of the mobile station ofthe first cell, and the base station of the second cell includes: aprocessor configured to execute a second process including: transmittinga null symbol by using a second resource of the data channel of thesecond cell, the second resource corresponding to the predeterminedresource unit, and the mobile station of the first cell includes: aprocessor configured to execute a third process including: receiving thecontrol signal transmitted by the base station of the first cell via thefirst resource, wherein the predetermined resource unit is aninterference reducing Control Channel Element (CCE) of a picocell. 2.The wireless communication system according to claim 1, wherein thepredetermined resource unit is one or more control channel elements, andthe first resource is a search space specific to the mobile station ofthe first cell.
 3. The wireless communication system according to claim1, further comprising a plurality of the first cells, wherein the secondprocess further includes calculating, for each of the first cells, aresource to which a predetermined resource unit of each of the firstcells is mapped, based on a wireless parameter of each of the firstcells, and transmitting, by a same subframe, a null symbol in all of theresources calculated at the calculating.
 4. The wireless communicationsystem according to claim 1, further comprising a plurality of the basestations of the first cells, wherein the first process includesnotifying the base station of the second cell of information needed tospecify a resource to which the predetermined resource unit is mappedand of information on the number of interfered mobile stations,performing transmission by a control channel for a mobile station byusing a predetermined resource unit in an overlapping region when aresource that is specific to the mobile station connected to acorresponding one of the first cells and serving as a decoding object ofthe mobile station overlaps the predetermined resource unit in aninterference reducing subframe for each of the first cells indicated byinterference reducing subframe information notified by the base stationof the second cell, and the second process includes setting aninterference reducing subframe for each of the first cells based on theinformation on the number of the interfered mobile stations notified bythe base stations of the first cells, and notifying the base stations ofthe first cells of the interference reducing subframe information, andtransmitting a null symbol in a resource to which a predeterminedresource unit of each of the first cells is mapped in the interferencereducing subframe for each of the first cells.
 5. The wirelesscommunication system according to claim 4, wherein the first processincludes notifying the base station of the second cell of information onthe number of mobile stations that are in a wireless channel state of apredetermined quality or lower, as the information on the number of theinterfered mobile stations.
 6. The wireless communication systemaccording to claim 4, wherein the second process includes setting aratio between the first cells with regard to the number of theinterference reducing subframes, based on a ratio between the firstcells with regard to a value indicated by the information on the numberof the interfered mobile stations.
 7. The wireless communication systemaccording to claim 4, wherein the second process includes setting theinterference reducing subframes of the respective first cells so thatthe interference reducing subframes do not overlap each other.
 8. Thewireless communication system according to claim 4, wherein the firstprocess includes notifying the base station of the second cell of awireless parameter of each of the first cells as information needed tospecify a resource to which the predetermined resource unit is mapped,and the second process includes specifying a resource to which apredetermined resource unit of each of the first cells is mapped, basedon information on the wireless parameter notified by each of the basestations of the first cells.
 9. The wireless communication systemaccording to claim 4, wherein the first process includes provisionallyselecting an aggregation level to be applied to a control channel for amobile station connected to each of the first cells, based on a wirelesschannel state of the mobile station, and when a target resource that isspecific to the mobile station corresponding to the provisionalaggregation level of the mobile station and that serves as a decodingobject does not overlap the predetermined resource unit, the firstprocess includes selecting and transmitting an aggregation level, forwhich the target resource overlaps the predetermined resource unit andwhich is equal to or greater than the provisional value, or selects anaggregation level, for which the target resource overlaps thepredetermined resource unit and which is smaller than the provisionalvalue, and transmitting the aggregation level by adjusting transmissionpower to be equal to or greater than a predetermined value.
 10. Thewireless communication system according to claim 1, wherein the firstprocess includes notifying the base station of the second cell of thenumber of interfered mobile stations, and setting a predeterminedresource unit according to information on an interference reducingresource unit notified by the base station of the second cell, and thesecond process includes adjusting the number of predetermined resourceunits based on the number of the interfered mobile stations notified bythe base station of the first cell, and notifying the base station ofthe first cell of information on an interference reducing resource unitbased on the number of the predetermined resource units.
 11. Thewireless communication system according to claim 10, wherein the secondprocess includes increasing the number of the predetermined resourceunits when the number of the interfered mobile stations is equal to orgreater than a predetermined value, and decreasing the number of thepredetermined resource units when the number of the interfered mobilestations is smaller than the predetermined value.
 12. The wirelesscommunication system according to claim 4, wherein upon receiving arequest for transmission of the null symbol from any of the basestations of the first cells, the second process includes transmitting anull symbol to a mobile station of the corresponding base station of thefirst cell.
 13. A base station of a first cell in a wirelesscommunication system that controls a transmission timing in each cell sothat a control channel of the first cell and a data channel of a secondcell temporally overlap each other, the base station comprising: aprocessor configured to execute a process including: notifying a basestation of the second cell of information used to specify a resource ofthe control channel of the first cell, the resource corresponding to apredetermined resource unit; and transmitting a control signal to amobile station of the first cell by using a first resource of thecontrol channel of the first cell, the first resource corresponding toat least a part of the predetermined resource unit and serving as adecoding object of the mobile station of the first cell, wherein thepredetermined resource unit is an interference reducing Control ChannelElement (CCE) of a picocell.
 14. A base station of a second cell in awireless communication system that controls a transmission timing ineach cell so that a control channel of a first cell and a data channelof the second cell temporally overlap each other, the base stationcomprising: a processor configured to execute a process including:receiving information used to specify a resource of the control channelof the first cell from the base station of the first cell, the resourcecorresponding to a predetermined resource unit; and transmitting a nullsymbol by using a second resource of the data channel of the second cellbased on the received information, the second resource corresponding tothe predetermined resource unit, wherein the predetermined resource unitis an interference reducing Control Channel Element (CCE) of a picocell.15. A mobile station that performs a communication in a wirelesscommunication system that controls a transmission timing in each cell sothat a control channel of a first cell and a data channel of a secondcell temporally overlap each other, the mobile station comprising: aprocessor configured to execute a process including: receiving a controlsignal transmitted by a base station of the first cell by using a firstresource of the control channel of the first cell, the first resourcecorresponding to at least a part of a predetermined resource unit andserving as a decoding object of the mobile station of the first cell,wherein the predetermined resource unit is an interference reducingControl Channel Element (CCE) of a picocell.
 16. A wirelesscommunication method implemented by a wireless communication system forcontrolling a transmission timing in each cell so that a control channelof a first cell and a data channel of a second cell temporally overlapeach other, the wireless communication method comprising: notifying, bya base station of the first cell, a base station of the second cell ofinformation used to specify a resource of the control channel of thefirst cell, the resource corresponding to a predetermined resource unit,by a processor; transmitting, by the base station of the first cell, acontrol signal to a mobile station of the first cell by using a firstresource of the control channel of the first cell, the first resourcecorresponding to at least a part of the predetermined resource unit andserving as a decoding object of the mobile station of the first cell, bythe processor; transmitting, by the base station of the second cell, anull symbol by using a second resource of the data channel of the secondcell, the second resource corresponding to the predetermined resourceunit, by the processor; receiving, by the mobile station of the firstcell, the control signal transmitted by the base station of the firstcell by using the first resource, by the processor, wherein thepredetermined resource unit is an interference reducing Control ChannelElement (CCE) of a picocell.